1. Field of the Invention
The present invention relates to a gas concentration measurement apparatus, and more particularly to a fault detection technology for detecting a fault in a gas concentration apparatus.
2. Background Art
A conventional NOx concentration sensor for detecting the NOx concentration in an exhaust gas is disclosed, for instance, by Japanese Patent Laid-open No. 2000-214130 (hereinafter referred to as “Patent Document 1”). In this NOx concentration sensor, a pump cell is positioned upstream of a measurement target gas chamber while a sensor cell is positioned downstream of the measurement target gas chamber. The pump cell and sensor cell both comprise a solid electrolyte and a pair of electrodes that are mounted on the surface of the solid electrolyte. When a predetermined voltage is applied between the electrodes, the pump cell discharges (pumps) oxygen out of the measurement target gas chamber and causes a current to flow in accordance with the amount of the discharged oxygen. The sensor cell, on the other hand, resolves NOx, which exists in the measurement target gas chamber, into nitrogen and oxygen. When a predetermined voltage is applied between the electrodes, the sensor cell discharges the oxygen from the measurement target gas chamber and causes a current to flow in accordance with the amount of the discharged oxygen.
First of all, the pump cell of the above NOx concentration sensor removes remaining oxygen from an exhaust gas that enters the measurement target gas chamber. When the exhaust gas reaches the sensor cell after oxygen removal, NOx in the exhaust gas is resolved into nitrogen and oxygen. The sensor cell then generates a current in accordance with the resulting amount of oxygen, that is, the amount of NOx in the exhaust gas. Therefore, the above NOx concentration sensor can measure the NOx concentration in the exhaust gas in accordance with the value of a current flow in the sensor cell.
At present, the NOx concentration sensor is regarded as an important sensor for controlling an internal combustion engine in accordance with the information detected by the sensor and guaranteeing the emission from an internal combustion engine. It is therefore necessary that the NOx concentration sensor properly function at all times. It is demanded that any fault existing in the NOx concentration sensor be accurately detected. Particularly, an open circuit is a fundamental fault and should be detected early.
As a fault detection method for the pump cell of the NOx concentration sensor, the current flowing when the air-fuel ratio indicates a known predefined operating state (in which an output is always generated when no abnormality exists) may be measured and compared against a judgment value. However, it is difficult to determine during a vehicle run whether the above predefined operating state prevails. Even if such determination is accomplished, no fault detection can be achieved until the predefined operating state arises.
If the output current continues to be zero, a common sensor fault detection method may be used to conclude that there is an open circuit in the sensor. In the NOx concentration sensor, however, the output currents of both the pump cell and sensor cell may be zero from the viewpoint of control. If, for instance, the air-fuel ratio is stoichiometric, the pump cell's output current (pump cell current) is zero. Even when there is an open circuit, the apparent output current does not continue to be zero due, for instance, to surrounding noise. Particularly, the output current of the sensor cell (sensor cell current) is as small as several hundred nA even when the sensor cell is properly functioning. It is therefore difficult to detect an open circuit by determining whether the output current remains zero.
A fault detection method for use with sensors other than the NOx concentration sensor may be applied to the NOx concentration sensor. For an O2 sensor, a fault detection method disclosed, for instance, by Japanese Patent Publication No. Hei 7-69288 (hereinafter referred to as “Patent Document 2”) is known. For an air-fuel ratio sensor, a fault detection method disclosed, for instance, by Japanese Patent No. 2505152 (hereinafter referred to as “Patent Document 3”) is known. The fault detection method disclosed by Patent Document 2 raises a voltage present at one end of the sensor to check for a resulting change in the DC current for open-circuit detection purposes. The fault detection method disclosed by Patent Document 3 operates an air pump to supply atmospheric air to the inside of a sensor housing while the exhaust gas air-fuel ratio is in a steady state, thereby forcibly changing the pump cell current. The actual pump cell current detected at the time of atmospheric air supply is used to determine whether the pump cell is faulty. Further, the detected output voltage change in a sensing cell is used to determine whether the sensing cell is faulty.
However, the fault detection method disclosed by Patent Document 2 cannot readily be applied to an NOx concentration sensor such as a pump-cell-based sensor. For the voltage-current characteristic exhibited by the pump cell, there is a limiting current region in which the current is substantially constant relative to the voltage, and the voltage applied to the pump cell is controlled so that the pump cell current is within the limiting current region. Therefore, even if the voltage at one end is raised as is the case with a method disclosed by Patent Document 2, the pump cell current remains virtually unchanged. If, on the contrary, the pump cell current increases in accordance with the voltage, the pump cell current exceeds the limiting current. In such an instance, oxygen molecules may be pumped out of a solid electrolyte to incur blackening of the solid electrolyte, thereby causing the pump cell to deteriorate.
The method disclosed by Patent Document 3 is applied to an air-fuel ratio sensor that uses a pump cell as is the case with the NOx concentration sensor. However, the method disclosed by Patent Document 3 has a disadvantage, which will now be described. To detect a fault, the method disclosed by Patent Document 3 supplies atmospheric air from the outside to forcibly vary the pump cell current (limiting current) and checks whether the output value of the pump is within a predetermined range. For fault detection purposes, it is therefore necessary that the air-fuel ratio be in a steady state. It means that the opportunity for fault detection is limited. The air-fuel ratio may continue to be unstable for an extended period of time depending on the operating status of an internal combustion engine. It is therefore preferred that sensor fault detection be achievable without regard to the air-fuel ratio. Further, the method disclosed by Patent Document 3 cannot restore the normal control state until atmospheric air leaves the sensor housing after fault detection, causing the internal air-fuel ratio to revert to the original value. The information detected by the NOx concentration sensor and air-fuel ratio sensor is important for internal combustion engine control. Therefore, the period during which the sensor's gas concentration measurement function is rendered inoperative subsequently to fault detection should be minimized.